The present invention relates generally to metal matrix composites, and more particularly to the use of shape memory alloys in metal matrix composites, and to a method of making such composites which employs powder metallurgical techniques.
Shape memory alloys are alloys which undergo temperature-dependent and/or load-dependent phase transformations from one solid phase to another solid phase. For instance, at a temperature below the alloy's transition temperature range, the solid phase is martensitic. Above the transition temperature range the alloy typically is in a body-centered cubic solid phase known as austenite. Such an alloy can be formed into a desired shape when in the austenitic phase and then heat-treated to remember that shape. If the alloy is subsequently deformed while in the martensitic state, it will regain the desired shape upon being heated to a temperature at which it becomes austenite.
Because of their ability to return to an original desired shape, shape memory alloys have been a major element of the smart materials and smart structures research and development effort. Many designs specify the monolithic application of these materials. However, some applications call for the embedding of shape memory alloys within structural components, in order, for example, to sense environmental changes and to control structural and mechanical responses. Currently, shape memory alloy wires are embedded in structural materials to meet these needs. This method of embedding shape-memory alloys into structural components is labor intensive and expensive. Furthermore, it would be desirable to provide a structural component which has a more uniform distribution of shape-memory properties throughout it than these components have.
Shape memory alloys have been processed using powder metallurgical techniques. For instance, powders of different shape memory alloys have been blended to form an alloy which has a transition temperature range somewhere between those of the individual powders. Shape memory alloy powders have also been blended with metal carbide powders to form a composite with the shape memory alloy forming the matrix and the metal carbide particles being dispersed throughout the matrix. There does not currently exist, however, a metal matrix composite suitable for structural applications which has a uniform distribution throughout its matrix of shape memory alloy particles.